Is Under Ninja hentai?
No
Rated R (17+ (violence & profanity)) on MyAnimeList and not tagged as Hentai.

TV · 2023 · 12 episodes · R - 17+ (violence & profanity)
Action, Seinen
Once believed to be extinct, ninjas have reappeared in contemporary Japan following the Pacific War. With a headcount rumored to be two hundred thousand, these deadly assassins are now tasked with covert operations, infiltrating a significant portion of the nation's public and private institutions in the process. Among these ninjas is Kurou Kumogakure, who often spends his days in a run-down apartment with no motivation to do much of anything. Some might consider him a useless vagrant who does nothing but steal beer from his neighbor. Nevertheless, the 17-year-old loser might actually be more dangerous than his demeanor suggests. Kurou's hidden potential is put to the test when he is given a new mission to infiltrate a local high school, but there is more to this assignment than he could ever predict. [Written by MAL Rewrite]
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